feat(dashboards): scheduler health strip, failing-source rollup, 24h activity sparkline, credential staleness nudge

D1 scheduler visibility: AppSetting last-tick stamp on every Beat tick +
GET /api/sources/schedule-status (last_tick_at/next_due_at/due_now/auto_sources)
+ SchedulerStatusBar on the Subscriptions tab (re-polled every 30s).

D2 failing-source rollup: ?failing=true on the sources list + FailingSourcesCard
on Downloads with per-source and bulk "retry" (re-runs the feed via /check).

D3 activity sparkline: GET /api/downloads/activity hourly buckets + CSS bar
chart by the stat chips (failures stacked in error color); refreshes on live poll.

D4 credential staleness: surface last_verified age + "re-verify recommended"
warning past 30d; also fixes the dead last_verified_at field-name mismatch so
the verification row renders at all.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/stores/sources.js

@@ -9,6 +9,7 @@ export const useSourcesStore = defineStore('sources', () => {
   const byArtist = ref(new Map())
   const loading = ref(false)
   const error = ref(null)
+  const scheduleStatus = ref(null)
 
   const allSources = computed(() => byArtist.value.get(null) ?? [])
 
@@ -70,6 +71,11 @@ export const useSourcesStore = defineStore('sources', () => {
     return await api.get('/api/artists/autocomplete', { params: { q: query, limit } })
   }
 
+  async function loadScheduleStatus() {
+    scheduleStatus.value = await api.get('/api/sources/schedule-status')
+    return scheduleStatus.value
+  }
+
   // FC-3c: trigger a download for one source. Returns {download_event_id,status}.
   const checkingIds = ref(new Set())
 
@@ -104,13 +110,14 @@ export const useSourcesStore = defineStore('sources', () => {
   }
 
   return {
-    byArtist, loading, error,
+    byArtist, loading, error, scheduleStatus,
     allSources,
     checkingIds,
     loadAll, loadForArtist,
     create, update, remove,
     checkNow,
     findOrCreateArtist, autocompleteArtist,
+    loadScheduleStatus,
     sourcesByArtistGrouped,
   }
 })